Technical Abstract: Insects form symbiosis with fungi widely, especially those feeding on leaf litter. As dead plant tissues provide poor quality diets which contain relatively high levels of indigestible lignin and cellulose, saprophytic fungi may increase nutrient availability by polysaccharide degradation. Although those inherited, obligate bacterial symbionts are well documented, those non-inherited, facultative fungal symbionts are relatively overlooked. The females of the leaf-rolling weevil Heterapoderopsis bicallosicollis, a specialist of Triadica sebifera, construct leaf-rolls that serve as retreats from which larvae feed internally. We found that fungi associated with leaf-rolls were not transported by the female but likely originated from the soil. To determine the effects of fungi on H. bicallosicollis development, fungal growth was reduced by a dry treatment. This treatment decreased adult weight and survival and prolonged larval duration significantly. We further tested the hypothesis that fungi degrade leaf-roll polysaccharides by a fungus inoculation experiment. Lignin, cellulose and soluble carbohydrate of leaf-rolls all decreased during fungi exposure. Three dominant fungi (Penicillium sp., Aspergillus sp. and Cladosporium sp.) decreased the levels of lignin, cellulose and soluble carbohydrate in inoculation experiments. We conclude that these saprophytic fungi form facultative associations with H. bicallosicollis and benefit weevil nutrition by polysaccharide decomposition. Our study highlights the significance of fungi in insect nutritional ecology.